1. Field of the Invention
The present invention relates to the field of trucks for railroad cars, and in particular, to steerable trucks for railroad cars.
2. The Prior Art
The wheels which are used on railroad trucks are, almost universally, formed with conical tapered profiles. That is, the diameters of the wheels decrease, with the portions having the smallest diameter facing outwardly, relative to the railroad car. In addition, rims, having overall diameters substantially greater than the largest diameter portion of the tapered wheel surface, are located at the innermost portions of the wheels, and placed on the truck axles and axle bearings, such that the distance between the rims of the wheels on an axle (collectively, “wheel set”) is slightly less than the distance between the inside edges of the rails.
In prior art conventional railroad trucks, the axles would be fixed relative to the truck. Typically, there would be provided two trucks situated adjacent the ends of a railroad car. Each truck is typically connected to the railroad car by a short, 14 or 16 inch diameter, cylindrical post extending downwardly from the carbody, which is received by a plate mounted generally centrally relative to the truck. The center post in such a typical prior art configuration would typically have been configured to permit a certain amount of pivoting of the truck, relative to the railroad car body. As a practical matter, the frictional forces generated by the surface contact area between the post and the plate, and the tremendous weight of the carbody, means that the amount of resistance to pivoting will be great. Thus, the friction between the truck bolster center plate and car body center bowl cause the truck to stick at any given position during travel.
However, steering forces between the wheels and rail are great enough to overcome this friction force. The steering forces cause the truck to yaw toward a position in the direction of travel. As the truck approaches the direction of travel, the steering forces decrease to a point no longer greater than the friction forces between the truck bolster center plate and car body center bowl. Since the friction forces between the plate and bowl are now greater than the steering forces, the frictions again cause the truck to stick in a position until the steering forces become great. This continuous entice energy process results in the truck never becoming truly aligned to the track. Additionally, this continuous entice energy process creates rolling resistance and wear on straight track as well as in curves.
This oscillation of the truck pivoting about the center post, describes a sinusoidal path along the track. As speed increases this phenomenon is more obvious and at higher speeds this becomes an unacceptable condition. This phenomenon is commonly called “hunting”. Hunting starts at low speeds and can lead to unacceptable lateral wheel force, acceleration, and frequency unless constrained. The instability transfers rolling energy into undesirable lateral energy which creates rolling resistance, lading and car damage, and wheel and track wear.
As a railroad car having such prior art trucks would enter a curve, the rails move from under the wheels. The radius of one wheel increases as the radius of the other decreases. The different diameters creates a larger circumference on one wheel and smaller on the other. The difference in circumference causes one wheel to travel further than the other for the same revolution. The difference in travel causes the wheels and axle to turn in the direction of the curve. Ideally, the large circumference matches the length of the outside rail and the small circumference the inside rail.
The natural tendency of a single axle wheel set, in a curve, is to assume a posture is which the axle “points” to the center of curvature of the curve. This movement of a single axle may be referred to as “going radial”. In a prior art two wheel set truck with fixed axles, the axles would not be free to assume this described posture independently of one another, and the truck as a whole would be forced to rotate about the center of the truck. This condition creates high forces on the wheel sets and the truck, increases wear on the truck components, and increases rolling friction, resulting in increased fuel consumption as a result of the additional energy which had to be expended to keep the railroad cars moving. Additionally, these high forces also wear the track and the wheels.
A typical prior art truck configuration would comprise two longitudinally extending (i.e., track-wise extending) side frames, with a transversely extending bolster attached to the side frames (the “three-piece truck”). The axles of the wheel sets would be mounted fore and aft of the bolster, with the axle ends being generally fixed relative to the side frames.
Even though nominally rigidly constructed, such a truck configuration would, under sufficient loading (such as during curves), deform. Typically, this deformation would take the form of the side frames, bolster and wheel-sets skewing relative to one another to form a parallelogram, as the forces exerted on the wheels push the axles to seek yawed positions through the curve. Such parallelogramming is believed to be a common cause of railroad car derailment at low speed in curves. In rigid frame trucks this parallelogramming does not occur.
Accordingly, it can be seen that making trucks rigid and mounting them rigidly to car bodies, in an effort to eliminate hunting, and providing truck pivoting and/or flexibility, to permit truck or axle yawing or steering in curves, can and have created a design impasse for the creation of an effective three piece truck. Frame bracing of the three piece trucks helps, but still does not give a satisfactory result on wheel life and track loads.
Numerous attempts have been made to produce trucks which satisfy the requirements for efficient rolling during both straight runs and curves. Such attempts have included the provision of resilient or elastic members in the side frames and/or bolsters, pivot-mounted axles and side frames with damping apparatus like shock absorbers, and various forms of cross-bracing and the like. Such prior art configurations typically have resulted in truck structures which are costly, heavy, and/or overly complex and prone to failure or requiring extensive maintenance and replacement of components.
One such attempt is illustrated in U.S. Pat. No. 5,249,530. The '530 patent discloses a steering apparatus that responds to high speed single rail vertical curve change so accurately that the truck would yaw as if it had detected a lateral curve. The car mass would continue in the original direction of travel creating high lateral force, which would lift the inside wheels from the rail. The alternating and repeated single rail vertical curves is referred to by the Association of American Rail Roads as the twist and roll regime.
To correct the problem an active lateral suspension steering system in combination with steering was conceived in U.S. Pat. No. 5,666,885. In the '885 patent, the lateral suspension movement was intended to retard or add steering in lateral acceleration regimes. The '885 invention has a prompting mechanism to facilitate turning, however the lateral suspension steering of the '885 patent is prone to high friction and not able to demonstrate the effects of steering combined with the lateral suspension.
Another such attempt to satisfy the requirements for efficient rolling during both straight runs and curves is illustrated in U.S. Pat. No. 5,918,546. The '546 invention incorporated a low friction lateral suspension steering that through lateral acceleration retards or adds steering. Although the mechanism provided the concept, it is too complex for practical application.